1. Field of the Invention
The present invention relates to thin film solar cell modules and methods of manufacturing them.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods. Group IIB-VIA compounds such as CdTe, Group IBIIIAVIA compounds and amorphous Group IVA materials such as amorphous Si and amorphous Si alloys are important thin film materials that are being developed.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn(1−x)Gax(SySe(1−y))k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Among the family of compounds, best efficiencies have been obtained for those containing both Ga and In, with a Ga amount in the 15-25%. Recently absorbers comprising Al have also been developed and high efficiency solar cells have been demonstrated using such absorbers.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. The device 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. The absorber film 12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)2 , is grown over a conductive layer 13 or a contact layer, which is previously deposited on the substrate 11 and which acts as the electrical ohmic contact to the device. The most commonly used contact layer or conductive layer in the solar cell structure of FIG. 1 is Molybdenum (Mo). If the substrate itself is a properly selected conductive material such as a Mo foil, it is possible not to use a conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. The conductive layer 13 may also act as a diffusion barrier in case the metallic foil is reactive. For example, foils comprising materials such as Al, Ni, Cu may be used as substrates provided a barrier such as a Mo layer is deposited on them protecting them from reaction with Se or S as well as with the other components of the material such as Cu, In and Ga.
After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stack is formed on the absorber film. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side. A variety of materials, deposited by a variety of methods, can be used to provide the various layers of the device shown in FIG. 1.
Thin film photovoltaic devices may be manufactured in the form of monolithically integrated modules where electrical interconnection of individual solar cells is achieved on a single substrate, such as a glass sheet, during the film deposition through repeated “scribing/depositing” steps and a module on a single substrate may be obtained. Alternatively thin film solar cells may be manufactured individually, physically separate from each other, and then connected in series, through use of soldering or conductive epoxies (as in standard Si solar cells) to obtain high voltage modules. In this case, solar cells often need to be large area, one dimension being more than 1″, typically more than 3″. Such large area requires deposition of finger patterns over the top conducting layer of the solar cell, such as the transparent layer 14 in FIG. 1.
Roll-to-roll or reel-to-reel fabrication of CIGS(S) type solar cells is attractive because of cost lowering potential and ease of manufacturing. In such an approach the CIGS(S) device structure of FIG. 1 may be formed on a conductive foil substrate such as a stainless steel or aluminum substrate, in a roll-to-roll fashion, and then individual solar cells may be cut from the roll which may be 0.3-1.2 meters (m) wide and 150-900 m long (approximately 1-4 ft wide and 500-3000 ft long). The cut cells may then be interconnected and packaged into modules. This interconnection step of prior-art approaches is labor intensive and complex and involves handling of a large number of individual solar cells. Present invention aims to simplify the interconnection process and reduce cost.